Backends: OpenGL3: Commented out extra tokens at end of #else directive (#2804)

diff --git a/examples/imgui_impl_opengl3.cpp b/examples/imgui_impl_opengl3.cpp
index 189814f..e1eee23 100644
--- a/examples/imgui_impl_opengl3.cpp
+++ b/examples/imgui_impl_opengl3.cpp
@@ -172,7 +172,7 @@
     gl_loader = "GLEW";
 #elif defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)
     gl_loader = "GLAD";
-#else IMGUI_IMPL_OPENGL_LOADER_CUSTOM
+#else // IMGUI_IMPL_OPENGL_LOADER_CUSTOM
     gl_loader = "Custom";
 #endif